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Published in final edited form as: J Phys Chem C Nanomater Interfaces. 2021;125(28):10.1021/acs.jpcc.1c03629. doi: 10.1021/acs.jpcc.1c03629

Neutron Scattering Investigations of the Global and Local Structures of Ammine Yttrium Borohydrides

Jakob B Grinderslev 1, Mikael S Andersson 2,3, Benjamin A Trump 4, Wei Zhou 5, Terrence J Udovic 6,7, Maths Karlsson 8, Torben R Jensen 9
PMCID: PMC10938370  NIHMSID: NIHMS1918225  PMID: 38487813

Abstract

Complex metal hydrides are a fascinating and continuously expanding class of materials with many properties relevant for solid-state hydrogen and ammonia storage and solid-state electrolytes. The crystal structures are often investigated using powder X-ray diffraction (PXD), which can be ambiguous. Here, we revisit the crystal structure of Y(11BD4)3·3ND3 with the use of neutron diffraction, which, in comparison to previous PXD studies, provides accurate information about the D positions in the compound. Upon cooling to 10 K, the compound underwent a polymorphic transition, and a new monoclinic low-temperature polymorph denoted as α-Y(11BD4)3·3ND3 was discovered. Furthermore, the series of Y(11BH4)3·xNH3 (x = 0, 3, and 7) were also investigated with inelastic neutron scattering and infrared spectroscopy techniques, which provided information of the local coordination environment of the 11BH4– and NH3 groups and unique insights into the hydrogen dynamics. Partial deuteration using ND3 in Y(11BH4)3·xND3 (x = 3 and 7) allowed for an unambiguous assignment of the vibrational bands corresponding to the NH3 and 11BH4– in Y(11BH4)3·xNH3, due to the much larger neutron scattering cross section of H compared to D. The vibrational spectra of Y(11BH4)3·xNH3 could roughly be divided into three regions: (i) below 55 meV, containing mainly 11BH4– librational motions, (ii) 55–130 meV, containing mainly NH3 librational motions, and (iii) above 130 meV, containing 11B–H and N–H bending and stretching motions.

Graphical Abstract

graphic file with name nihms-1918225-f0005.jpg

INTRODUCTION

Metal borohydrides and derivatives thereof are a continuously expanding class of materials, and numerous new compositions have been reported in the past decade.1–6 Besides high hydrogen densities, many of these materials exhibit other interesting properties such as magnetism,7,8 luminescence,9–11 or ionic conductivity.12–19 Ammine metal borohydrides have been particularly investigated due to dihydrogen interactions, N–Hδ+···–δH–B, which can facilitate the release of hydrogen at low temperatures.1,20 Recently, these dihydrogen interactions have received new interest due to their influence on dynamics and crystal structures,4,21,22 and it has been suggested that these interactions can facilitate fast ionic conductivity, forming the basis for a new type of solid-state electrolyte.15,16,23

Ammonia derivatives are known for the majority of the metal borohydrides,1 and the largest series of compositions and polymorphs among ammine metal borohydrides are observed for Y(BH4)3·xNH3, with x = 7, 6, 5, 3, 2 (α- and β-polymorph), and 1.24,25 The structures possess an intriguing crystal chemistry dependent on the number of ammonia molecules, varying from structures built from complex ions (x = 5, 6, and 7) to molecular structures (x = 3), one-dimensional chains (x = α−2 and β−2), and structures built from two-dimensional layers (x = 1). Note that the crystal structure for x = 5 and both the structure and composition for x = 3 were recently revised using a combination of powder X-ray diffraction (PXD), density functional theory (DFT) calculations, 11B nuclear magnetic resonance (NMR) spectroscopy, and thermal analysis.25,26

Ab initio structural solution from PXD data is challenging, in particular, for complex metal hydrides. These materials can exhibit crystallographic difficulties such as multiphase samples, anisotropic peak broadening, superstructures, pseudosymmetry, weakly scattering elements (H), and polymorphic transitions, and chemical reactions can occur during heating.27,28 Furthermore, isoelectronic ions and molecules, such as BH4−, NH3, and NH4+, and metals with a similar atomic number, for example, in the series of rare-earth metals, are very difficult to distinguish with X-ray scattering.4,26 Indexing is often performed using the “decomposition-aided indexing” approach, particularly important for multiphase samples.28 Correct indexing and space group determination are important for subsequent structural solution, and estimating the correct chemical composition can be crucial for choosing the right unit cell.29,30 The crystal structures of complex metal hydrides are often solved from direct space modeling, but incomplete or wrong structural models and/or wrong chemical compositions may still yield a convincing fit to the measured PXD data.30 DFT calculations can be an important tool for validation of crystal structure models and compositions, while also correcting the space group and determining the H positions.28 DFT calculations ensure that the structure corresponds to a local potential energy minimum and that the interatomic distances are balanced. However, the technique has some potential pitfalls. Complex hydrides often display disorder, and structures solved at room temperature may display dynamics that are not accounted for in the DFT calculations, which are typically performed at 0 K.27 Diffraction data provide an average structure over time and space, whereas DFT calculations provide a static structure with minimal energy, so the unit cell parameters are often constrained to the experimental values.29 Crystallographic models can be complemented by experimental information obtained from other methods, for example, thermal analysis, quasielastic neutron scattering, and vibrational and NMR spectroscopy, which can provide information on the composition and the local environment of the structure, recently demonstrated from the revised structure of β-KB3H8.31,32

The composition and crystal structure of Y(BH4)3·3NH3 have previously been reported as “Y(BH4)3·4NH3”.24,33 The initial structural model was suggested to consist of a complex [Y(NH3)4(BH4)2]+ octahedral cation and a [BH4]− counteranion.33 However, 11B-NMR data on a purer sample (without impurities of LiBH4) revealed that all BH4− anions experience a similar local environment, and a new crystal structure model was proposed, consisting of neutral molecular [Y(NH3)4(BH4)3] complexes.24 DFT calculations revealed a lower energy for the second structure, but with a volume expansion of 11–18% compared to the experimental unit cell.24 In a subsequent study, the isostructural Ce-analogue was investigated, and a revised composition and new structural model were proposed, but with the same orthorhombic unit cell and space group symmetry Pna21.26 Thermogravimetric analysis of Ce(BH4)3·6NH3 revealed that three NH3 equivalents were released upon heating to 383 K, corresponding to the composition Ce(BH4)3·3NH3. However, Rietveld refinement of the measured PXD data using the previously reported structural model with the composition “Ce(BH4)3·4NH3” provided a convincing fit. This suggests an incorrect composition of the previously reported structural model, and further analysis of the data resulted in the revised composition and structural model Ce(BH4)3·3NH3. DFT structural optimization revealed that this new structural model has a lower energy and a 5% compression compared to the experimental structure, consistent with the exclusion of thermal expansion. This was later confirmed also for the isostructural rare-earth series, where the release of four NH3 from M(BH4)3·7NH3 (M3+ = La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, and Tm) resulted in the formation of M(BH4)3·3NH3.25

Thus, three different structural models and two compositions Y(BH4)3·xNH3 (x = 3 or 4) have been proposed for the same compound, which all provide a convincing fit to the observed PXD data. This has prompted the present investigation of an isotopically enriched compound, Y(11BD4)3·3ND3, with powder neutron diffraction (PND). Furthermore, we characterize Y(11BH4)3·xNH3 (x = 0, 3, and 7) and the deuterated ND3 analogues with inelastic neutron scattering (INS) and Fourier transform infrared spectroscopy (FTIR), which provide insights into the local structure of 11BH4− and NH3 or ND3 and on the hydrogen dynamics in the compounds. This work demonstrates how multiple characterization techniques may be necessary to obtain the right composition and verify the structural models.

EXPERIMENTAL SECTION

Sample Preparation.

Y(11BH4)3 and Y(11BD4)3 were prepared according to previously published procedures.8,34 Yttrium metal (99.9%, Sigma Aldrich35) was hydrogenated by heating from room temperature to 673 K, with a heating rate of ΔT/Δt = 5 K/min and an initial pressure of p(H2) = 140 bar or p(D2) = 40 bar. The samples were subsequently cooled to room temperature, and the pressure was released. The resulting YH3 or YD3 was ball-milled using a Fritsch Pulverisette no. 6 in an 80 mL tungsten carbide vial together with tungsten carbide-coated steel balls (d = 10 mm) in a ball-to-powder mass ratio of 10:1, with a ball milling program of 10 min at 350 rpm (5.83 Hz), followed by a 2 min break. This sequence was repeated 10 times. The as-milled yttrium hydride was added to a round-bottomed flask with a valve outlet. Boron-11-enriched dimethyl sulfide borane (S(CH3)2·11BH3 or S(CH3)2·11BD3, 10 mol/L, Katchem) was added to the powder in the molar ratio 4.5:1 (50% excess of S(CH3)2·11BH3) and diluted to a 5 mol/L solution with toluene (anhydrous, Sigma-Aldrich). The reaction mixture was left to stir at 318 K for 7 d. Subsequently, the solvent was removed by filtration, and the powder was washed twice with toluene. The dry powdered Y(11BH4)3·S(CH3)2 or Y(11BD4)3·S(CH3)2 was transferred to Schlenk tubes and heated to 413 K in an argon atmosphere for 2 h, followed by 2 h under dynamic vacuum (p ∼ 10−4 bar), resulting in Y(11BH4)3 or Y(11BD4)3.

  1. Y(11BH4)3 was reacted with anhydrous NH3 or ND3 gas at ∼1 bar (Sigma Aldrich) and 253 K for 2 h and subsequently dried under vacuum for 10 min. The resulting white powders were identified by PXD as Y(11BH4)3·7NH3 or Y(11BH4)3·7ND3. Y(11BH4)3·3NH3 was prepared by thermal treatment of Y(11BH4)3·7NH3 at 333 K under vacuum for 3 h.

  2. Y(11BD4)3 was reacted with anhydrous ND3 gas at ∼1 bar (Sigma Aldrich) and 253 K for 2 h, and the resulting white powder was identified by PXD as Y(11BD4)3·xND3 (x = 5, 7). Y(11BD4)3·3ND3 was prepared by thermal treatment of the powder at 333 K under vacuum for 3 h.

All reagents and starting materials are air- and moisture-sensitive. All sample handling and preparation are performed in an inert atmosphere in a glovebox or using Schlenk techniques.

Synchrotron Radiation PXD.

Synchrotron radiation powder X-ray diffraction (SR PXD) data were collected for Y(11BH4)3·7NH3 at room temperature at beamline I11 at the Diamond Light Source, Oxford, UK on a wide-angle position sensitive detector based on Mythen-2 Si Strip modules with λ = 0.826460 Å. In situ time-resolved SR PXD data were collected for Y(11BH4)3·3NH3 at the Swiss-Norwegian beamline BM01A at the European Synchrotron Radiation Facility (ESRF), Grenoble, France, on a Dectris Pilatus 2 M area detector with λ = 0.686630 Å.36 The samples were initially cooled to 113 K and measured upon heating to 484 K using a heating rate of 6 K/min using an Oxford cryostream. The samples were packed in 0.5 mm borosilicate capillaries and sealed in an argon atmosphere.

Powder Neutron Diffraction.

PND measurements were conducted on 0.1833 g of Y(11BD4)3·3ND3 at the high-resolution neutron powder diffractometer BT1 at the NIST Center for Neutron Research (NCNR). The data were collected using a Ge(311) monochromator with an in-pile 60′ collimator, corresponding to a neutron wavelength of 2.0775 Å. The samples were sealed with an indium o-ring into a vanadium sample can inside a dry He glovebox. Measurements were conducted at 10 and 300 K while mounted on a bottom-loaded, closed-cooling refrigeration system.

Structure Solution and Refinement.

High-resolution SR PXD data and PND data were used for structural refinement, performed using the Rietveld method as implemented in the software Fullprof.37 The 11BH4− or 11BD4− and NH3 or ND3 groups were treated as rigid bodies. The structures of Y(BH4)3·xNH3 (x = 3 and 7) were used as starting points for Rietveld refinements.24–26 The background was described by linear interpolation between selected points, while Pseudo-Voigt profile functions were used to fit the diffraction peaks. Structures were checked for higher symmetry using the software Platon.38

The new low-temperature polymorph α-Y(11BD4)3·3ND3 was indexed in the PND data using the software FOX.39 Structural solution starting from a random configuration was unsuccessful, but a starting configuration was built based on the similarity with β-Y(11BD4)3·3ND3. This model was used for subsequent Rietveld refinements as described above.

Inelastic Neutron Scattering.

The INS experiments were carried out using the Filter-Analyzer Neutron Spectrometer at the NCNR40 with the Cu(220) and PG(002) monochromators with pre- and postcollimations of 20′ of arc, resulting in a full-width-at-half-maximum energy resolution of about 3% of the neutron energy transfer. The INS measurements were performed at 5 K for the Y(11BH4)3, Y(11BH4)3·3NH3, Y(11BH4)3·3ND3, Y(11BH4)3·7NH3, and Y(11BH4)3·7ND3 samples. For these measurements, about 0.3 g of the powder sample was evenly distributed into an Al-foil pouch, which was wrapped into an annulus and placed inside a sealed cylindrical Al sample cell. The preparation steps were carried out in a dry He glovebox.

Fourier Transform Infrared Spectroscopy.

FTIR spectroscopy was measured on an IRSpirit instrument from Shimadzu equipped with a diamond attenuated total reflectance crystal, situated within an Ar-filled glovebox. A total of 32 scans were collected and averaged in the range 500–4000 cm−1 with a spectral resolution of 4 cm−1.

DFT Calculations.

First-principles DFT calculations were performed using the Quantum ESPRESSO package,41 using Vanderbilt-type ultrasoft pseudopotentials with Perdew–Burke–Ernzerhof exchange correlations. A cutoff energy of 544 eV and appropriately chosen k-point meshes were used and found to be enough for the total energies to converge within 0.01 meV/atom. First, all crystal structures were fully optimized with respect to the atomic coordinates. For comparison with the experimental INS data, phonon densities of states (PDOSs) were calculated for the low-temperature structures using the finite differences method and were appropriately weighted to account for the total neutron scattering cross sections of the different elements. To assist in the interpretation of the FTIR data, we performed density functional perturbation theory (DFPT) calculations42 on the room-temperature structures and predicted infrared absorption cross sections. In the DFPT calculations, a larger cutoff energy of 1088 eV and denser k-point meshes were used to ensure adequate accuracy.

RESULTS AND DISCUSSION

Synthesis and Initial Analysis of the Products.

Anhydrous ammonia gas immediately reacts exothermically with yttrium borohydride to form ammine yttrium borohydrides, and a significant expansion of the powder is observed. The powdered products were identified by PXD as Y(11BH4)3·7NH3 and Y(11BH4)3·3NH3 or the deuterated analogues (Figures S1 and S2).24,25

Crystal Structure Analysis.

Y(11BD4)3·3ND3 has been measured by PND at 300 and 10 K. Analysis of the PND data revealed that Y(11BD4)3·3ND3 undergoes a polymorphic transition during cooling to 10 K (Table 1). Rietveld refinements of α- and β-Y(11BD4)3·3ND3 are shown in Figure 1. The structures were further validated using DFT calculations, which only resulted in minor changes in the atomic positions and no significant changes in the unit cell volume.

Table 1.

Crystallographic Data Obtained by Rietveld Refinement of PND Data

compound space group a (Å) b (Å) c (Å) β (°) V (Å3) T (K)
α-Y(11BD4)3·3ND3 P21 7.0165(6) 11.349(1) 11.853(1) 90.100(9) 943.8(1) 10
β-Y(11BD4)3·3ND3 Pna21 12.214(4) 7.082(2) 11.359(4) 982.6(6) 300
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Figure 1.

Figure 1.

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Rietveld refinements of PND data of α-Y(11BD4)3·3ND3 (P21) at 10 K and β–Y(11BD4)3·3ND3 (Pna21) at 300 K, showing experimental (black line) and calculated (red line) PND patterns and a difference plot below (blue line). Bragg peak positions are shown as black lines. Final discrepancy factors for α-Y(11BD4)3·3ND3: Rp = 2.19%, Rwp = 2.62% (not corrected for background), Rp = 9.28%, Rwp = 6.26% (conventional Rietveld R-factors), RBragg(α-Y(11BD4)3·3ND3) = 2.24%, and global χ2 = 1568. Final discrepancy factors for β–Y(11BD4)3·3ND3: Rp = 2.38%, Rwp = 2.80% (not corrected for background), Rp = 17.4%, Rwp = 7.74% (conventional Rietveld R-factors), RBragg(β-Y(11BD4)3·3ND3) = 2.75%, and global χ2 = 930.

Crystal structure of β-Y(11BD4)3·3ND3.

Three different crystal structures have been reported for the isostructural compounds M(BH4)3·3NH3 (M = Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, and Tm), originally reported with the composition “M(BH4)3·4NH3”.24–26,33 The revised crystal structure of M(BH4)3·3NH3 has been solved by a combination of PXD, 11B NMR, and DFT calculations. The compound has been investigated here by PND at 300 K (Figure 1) using an isotope-enriched sample, Y(11BD4)3·3ND3. The DFT-optimized Ce(BH4)3·3NH3 structure was used as a starting point for the refinements of the PXD (113 K) and PND data (300 K).26 Rietveld refinements mainly resulted in a decrease in the M-B and M-N distances, as expected from the difference in ionic radii, r(Y3+) = 0.900 Å and r(Ce3+) = 1.01 Å.43 The coordination of the BD4− groups to the metal remained the same, but there were minor changes in the atomic positions.

Subsequent analysis for higher symmetry using Platon suggests the space group Pnma if the D (H) positions are ignored and with small changes in the BD4− and ND3 positions. This results in two of the BD4− and two of the ND3 groups being crystallographically equivalent, in contrast to all ligands being crystallographically independent in the space group Pna21. For comparison, there are 28 independent atomic positions in the lower symmetry space group Pna21 and 17 for the higher symmetry space group Pnma. Subsequent structural solution and Rietveld refinement (Figures 1 and S3) result in the best fit to the PND data using the space group Pna21, which are also expected due to more free parameters. The two models were further compared using the discrepancy equation Q = D + a·p from Fullprof, where D is the deviance, a = 2, and p is the number of parameters. This accounts for both the goodness of fit and the number of parameters used to achieve that fit and results in Q = 1864 for Pna21 and Q = 2700 for Pnma, thus favoring the lower symmetry Pna21, which is used for further characterization. However, the differences between the two structures are relatively small and may depend on the specific metal ion in M(BH4)3·3NH3 and the temperature, suggesting that the space group Pnma may be a high-temperature polymorph.

The refined structure of β-Y(11BD4)3·3ND3 at 300 K is shown in Figure 2 (left). One unique Y3+ is coordinated by three ND3 molecules and three terminal 11BD4− groups, providing neutral [Y(ND3)3(11BD4)3] octahedral complexes with the ligands arranged in a facial geometry. The molecular complexes form hexagonal patterns in the ab plane and are stacked in the order ABAB along the c-axis. The Y–11B distances are in the range of 2.41–2.62 Å, and the Y–N bond distances are in the range of 2.52–2.54 Å. The three 11BD4− groups coordinate to Y3+ as tridentate ligands (κ3); thus, Y3+ has a coordination number of 12. The shortest dihydrogen interaction, 11B–Dδ−···+δD–N, is intermolecular and with a distance of 2.01 Å.

Figure 2.

Figure 2.

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Crystal structures of β-Y(11BD4)3·3ND3 and α-Y(11BD4)3·3ND3. Color scheme: Y3+ (blue), B (light blue), N (red), and D (gray). Blue polyhedra show local coordination of Y3+, while light blue tetrahedra are BD4−.

Crystal structure of α-Y(11BD4)3·3ND3.

A new low-temperature polymorph has been identified by PND for Y(11BD4)3·3ND3 upon cooling to 10 K (Figure 1), where new Bragg reflections that are not included in the orthorhombic Pna21 unit cell appear. The refinement of the PXD data at 113 K (Figure S2) suggests that the polymorphic transition occurs below this temperature or that the transition is not detectable by PXD. The refined structure of α-Y(11BD4)3·3ND3 in a monoclinic unit cell with the space group symmetry P21 is shown in Figure 2 (right) and is a substructure of the space group Pna21. The number of formula units (Z) is 4, similar to β-Y(11BD4)3·3ND3, but the multiplicity of the crystallographic Wyckoff sites is 2. Thus, there are two crystallographically distinct [Y(ND3)3(11BD4)3] octahedral units, but the geometry of the octahedra remains similar to β-Y(11BD4)3·3ND3, with the ligands arranged in a facial geometry. The molecular complexes form hexagonal patterns in the ac plane and are stacked in the order ABAB along the b-axis. The Y–11B distances are in the range 2.44–2.70 Å, which is slightly longer than that for β-Y(11BD4)3·3ND3, while the Y–N bond distances are slightly shorter in the range 2.31–2.54 Å. The coordination of the three 11BD4− groups are similar, and they coordinate to Y3+ as tridentate ligands (κ3), resulting in a coordination number of 12 for Y3+.

The monoclinic distortion of β- to α-Y(11BD4)3·3ND3 does not appear to be caused by an orientational ordering as has been reported for other borohydride-based compounds at low temperatures, for example, MBH4 (M = NH4+, Na+, K+, Rb+, and Cs+),21,22,44 as the refinements of the β-Y(11BD4)3·3ND3 polymorph suggests that the structure is ordered at 300 K. The slight distortion may rather be a consequence of other interactions. An analysis of short heterogeneous dihydrogen interactions, N–Hδ+...−δH–B (<2.4 Å), reveals that there are more interactions in α-Y(11BD4)3·3ND3 (4.83 dihydrogen bonds per NH3) compared to β-Y(11BD4)3·3ND3 (3.67 dihydrogen bonds per NH3). Alternatively, the transition may be a consequence of the shrinking unit cell lattice, which can result in shorter repulsive homopolar dihydrogen interactions, B–Hδ−...−δH–B, between neighboring BH4− groups, which have previously been shown to lower the symmetry in perovskite-type metal borohydrides.10,45

Inelastic Neutron Scattering.

The INS spectra measured at 5 K are presented in Figure 3 along with the corresponding DFT-calculated spectra for α-Y(11BH4), α-Y(11BH4)3·3NH3, and Y(11BH4)3·7NH3. The INS spectra reflect the PDOSs weighted by the neutron scattering cross sections of the different elements involved. As the relatively large scattering cross section of H dwarfs that of other elements, the INS spectra of hydrogenous compounds are dominated by all the normal mode vibrations involving hydrogen, with the most intense scattering features associated with the bands having the largest hydrogen vibrational amplitudes.

Figure 3.

Figure 3.

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INS spectra for (a) α-Y(11BH4)3, (b) α-Y(11BH4)3·3NH3 and α-Y(11BH4)3·3ND3, and (c) Y(11BH4)3·7NH3 and Y(11BH4)3·7ND3 measured at 5 K, as well as the corresponding DFT-calculated spectra for (a) α-Y(11BH4)3, (b) α-Y(11BH4)3·3NH3, and (c) Y(11BH4)3·7NH3. Spurious peaks are indicated with an “*”.

The experimental and calculated spectra are in good agreement with each other, which further confirms the validity of the proposed structural models. The spectra contain a few spurious peaks, indicated with an “*” in Figure 3. Further information about the spurious peaks is given in Supporting Information. A list of all the vibrational frequencies as determined by the DFT calculations of the low-temperature structures and animations of the corresponding vibrational motions are provided in Supporting Information.

Figure 3a shows the INS spectrum for α-Y(11BH4)3, together with the corresponding calculated spectrum. A comparison between the experimental and calculated spectra suggests that the spectra may be divided into two energy regions, related to various librational motions of the 11BH4− anion (below 130 meV) and various bending motions of the 11B–H bond of 11BH4− (above 130 meV). Note that stretches of the 11B–H bond of the 11BH4− anion are expected to be located above 280 meV, thus outside of the energy range probed here. This spectra compare well to those reported for other metal borohydrides, for example, the different polymorphs of Mg(BH4)2 (α, β, and γ), where the BH4– also acts as bridging bidentate ligands.46,47

Figure 3b shows the INS spectra for α-Y(11BH4)3·3NH3 and α-Y(11BH4)3·3ND3 and the calculated spectrum for α-Y(11BH4)3·3NH3. Considering first the fully protonated material, α-Y(11BH4)3·3NH3, a comparison between the experimental and calculated spectra suggests that the spectra may be divided into three energy regions: below 60 meV, 60–130 meV, and above 130 meV. The region below 60 meV is mainly dominated by librational motions of the 11BH4− anions. This region also contains bands related to Y–11BH4 and Y–N stretching motions, which, due to the movement of the whole 11BH4− anion or NH3 molecule, are observable in the INS spectra. In the spectrum of α-Y(11BH4)3·3ND3, scattering from the ND3 molecules will be substantially attenuated due to the more than an order-of-magnitude lower neutron scattering cross section for D. Comparing the α-Y(11BH4)3·3NH3 and α-Y(11BH4)3·3ND3 spectra and the calculated spectrum suggests that the peak at ≈24 meV is related to NH3 librations, while the peak at ≈36 meV is related to Y–N stretches. The spectrum is completely dominated by scattering from NH3 librational motions in the 60–130 meV range as observed by the disappearance of all features in the α-Y(11BH4)3·3ND3 spectrum in this energy range. Above 130 meV, the spectrum of α-Y(11BH4)3·3NH3 is dominated by three peaks at ≈136, 145, and 155 meV. The first two peaks are related to bending motions of the 11B–H bond in the 11BH4− anion, while the peak at 155 meV contains both 11B–H and N–H bending motions. This is observed as a loss of intensity for this peak in the α-Y(11BH4)3·3ND3 spectrum.

Figure 3c shows the INS spectra for Y(11BH4)3·7NH3 and Y(11BH4)3·7ND3 and the calculated spectrum for Y(11BH4)3·7NH3. Similar to the spectrum for α-Y(11BH4)3·3NH3 (Figure 3b), that for Y(11BH4)3·7NH3 may be divided into three energy regions: below 55 meV, 55–130 meV, and above 130 meV. The DFT calculations indicate significant van der Waals interactions in Y(11BH4)3·7NH3, which is challenging to describe accurately. This primarily affects the lower-energy excitations. The calculations indicate that the region below 55 meV is dominated by librational motions of the 11BH4− anions, while also containing Y–11BH4 and Y–N stretches. This is reflected in the comparison between the Y(11BH4)3·7NH3 and Y(11BH4)3·7ND3 spectra. One clear change is observed around 34 meV, which is assigned to Y–N stretching motions. Above 55 meV, the spectrum is dominated by librational motions of the NH3 molecules, and above 130 meV, three peaks can be identified at 136, 155, and 200 meV. The peak at 136 meV is related to 11B–H bending motions, the peak at 155 meV is related to both 11B–H and N–H bending motions, and the peak at 200 meV is related to N–H bending motions.

A comparison of the INS spectra above 130 meV for α-Y(11BH4)3, α-Y(11BH4)3·3NH3, and Y(11BH4)3·7NH3 shows that the changing NH3 content only has a minor effect on the observed vibrational modes (above 130 meV), as the 11B–H bending motions are not very sensitive to changes in the local environment. In contrast, librational motions are known to be more sensitive to changes in the local environment, as observed for the compounds here, where the low-energy librational modes of the 11BH4− anions exhibit a shift toward lower energies in the INS spectra with increasing NH3 content. This shift is related to the strength of the interaction between the 11BH4− anion and its surroundings, with stronger interactions resulting in higher BH4− librational energies. The local environment of 11BH4− in the three compounds is illustrated in Figure S4. In α-Y(11BH4)3, the 11BH4− acts as a bridging ligand between two Y3+ cations, interacting strongly and resulting in relatively high librational energies. In α-Y(11BH4)3·3NH3, the 11BH4− is only coordinated to one Y3+, while also surrounded by neutral NH3 molecules, resulting in overall weaker interactions and lower librational energies. In Y(11BH4)3·7NH3, the 11BH4− is surrounded entirely by NH3 molecules, resulting in even weaker interactions and the lowest librational energies compared to α-Y(11BH4)3 and α-Y(11BH4)3·3NH3. Besides markedly affecting the BH4− librational energies, the nature and varying strength of the interactions between the 11BH4− anion and its local environment for the range of Y(11BH4)3·xNH3 compounds will also likely have a dramatic influence on the 11BH4− rotational potentials, leading to expected wide variations in 11BH4− reorientational mobilities, an intriguing topic for future dynamics investigations.

Fourier Transform Infrared Spectroscopy.

FTIR data of the investigated samples are shown in Figure 4. The DFPT method was used to calculate and predict the absorption energies and cross sections for the room-temperature structures, which were used for the subsequent assignment of the vibrational bands. In general, DFPT calculates similar absorption energies to those experimentally determined and correctly predicts the splitting of the absorption bands (Figure S5). However, the relative intensities of the absorptions are not accurately determined by this method. A list of all the vibrational frequencies as determined by the DFPT calculations and animations of the corresponding vibrational motions are provided in Supporting Information.

Figure 4.

Figure 4.

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FTIR spectra obtained at room temperature. The lines indicate the assignment to the different motions.

Y(11BH4)3 shows absorption bands that can be assigned to 11B–H bending (∼125–155 meV) and 11B–H stretching (∼260–325 meV) motions. The hydrogen-containing samples, Y(11BH4)3·xNH3 (x = 3 and 7), show absorption bands that can be assigned to NH3 rocking (∼60–85 meV), 11B–H bending (∼125–155 meV), N–H bending (∼150–205 meV), 11B–H stretching (∼260–315 meV), and N–H stretching motions (∼390–420 meV). This agrees well with previous assignments in the literature, except for the NH3 rocking bands, which have previously been assigned to metal-nitrogen stretching motions for ammine metal borohydrides.24–26,48 The assignment to NH3 rocking agrees well with the literature on other ammine metal complexes.49

The splitting of the 11BH4− absorption bands is related to the specific coordination mode, which can be either mono-, bi-, or tridentate or it may act as a counterion.50,51 The absorption bands in Y(11BH4)3 are consistent with bridging bidentate 11BH4− groups and are similar to those observed in M(BH4)2 (M = Mg and Mn).48 In β-Y(11BH4)3·3NH3, the 11B–H stretches are significantly split, and the absorption band at 301 meV (2430 cm−1) corresponds to the terminal 11B–H, while the absorption bands at around 278 meV (2240 cm−1) mainly correspond to the 11B–H facing Y, consistent with a tridentate coordination.50,51 In Y(11BH4)3·7NH3, the 11BH4− can be considered as a counterion, and the splitting of the absorption mode at 278 meV (2240 cm−1) may be due to the dihydrogen interactions and the minor differences in the local environment for the three crystallographically independent 11BH4− groups.24

The samples containing deuterated ND3, Y(11BH4)3·xND3 (x = 3 and 7), have a significant shift toward lower energies for the N–D bands, and the absorptions can be assigned to ND3 rocking (<70 meV), N–D bending (∼110–130 meV), and N–D stretching (∼300–315 meV). The N–D stretching and bending bands overlap with the corresponding 11B–H bands. Introducing ND3 only results in minor differences of the 11B–H bands.

The interpretation of the FTIR spectra agrees well with that of the INS spectra and provides additional information on the ND3 vibrations. However, it is not possible to directly compare the intensities due to the differences in scattering cross sections and selection rules. In the case of INS, the scattering is clearly dominated by H, and as a result, the much weaker scattering from other elements will only contribute to the background.

CONCLUSIONS

Three different crystal structures and two compositions have been reported for the same compound, and here, we confirm the most recently proposed structure of Y(BH4)3·3NH3 with PND data, which also allowed for determination of the D (H) positions. Upon cooling to 10 K, the orthorhombic high-temperature polymorph, β-Y(11BD4)3·3ND3, undergoes a transition to a new monoclinic polymorph, α-Y(11BD4)3·3ND3, with two crystallographically independent neutral [Y(ND3)3(11BD4)3] complexes in the unit cell. The polymorphic transition is likely due to both attractive N–Hδ+...−δH–B and repulsive B–Hδ−···−δH–B dihydrogen interactions, and analysis of the PXD data at 113 K suggests that the polymorphic transition occurs below this temperature. In both structures, the ligands are arranged in a facial geometry and with tridentate 11BD4 coordination, resulting in a coordination number of 12 for Y. INS and infrared spectroscopy were used to characterize the local environment of 11BH4− and NH3 and provided new insights into the H-dynamics in Y(11BH4)3·xNH3 (x = 0, 3, and 7). Furthermore, the ND3 analogues were investigated to isolate the contribution from the 11BH4− anions in the INS spectra. This allowed a rough division of the INS spectra into three energy regions, (i) 11BH4− librations below 55 meV, (ii) NH3 librations between 55 and 130 meV, and (iii) 11B–H and N–H bending and stretching motions above 130 meV. A shift toward lower energies was observed for the 11BH4− librational modes with increasing NH3 content in the structures, due to a decreasing interaction between 11BH4− and its surroundings upon going from x = 0 to 3 to 7 in Y(11BH4)3·xNH3. This work demonstrates the importance of employing a multitechnique approach, including selective isotopic enrichment of samples, for the identification of the correct composition and structure of novel compounds.

Supplementary Material

Phonon Animation Files
Supporting Info

ACKNOWLEDGMENTS

The work was supported by Nordforsk via the project Functional Hydrides—FunHy (no. 81942), the Danish Council for Independent Research, Technology and Production (SOS-MagBat, 9041-00226B), CarlsbergFondet, and the Danish Natural Science Research Councils (DanScatt). Funding from the Danish Ministry of Higher Education and Science through the SMART Lighthouse is gratefully acknowledged. M.S.A. would like to acknowledge support from the Swedish Research Council (grant no. 2017-06345). M.K. acknowledges support from the Swedish Research Council (grant no. 2016-06958) and the Barbro Oshers Pro Suecia Foundation (grant nos. 2018-0069 and 2020-0034). DFT calculations were performed in part on the NIST Enki HPC cluster. We thank the Swiss Norwegian beamline BM01 at ESRF and the local contact Iurii Dovgaliuk for assistance during the experiments and beamline I11 at the Diamond Light Source and the local contact Stephen Thompson for assistance with data collection. We also acknowledge the use of the high-resolution powder diffractometer and Filter-Analyzer Neutron Spectrometer at the NIST Center for Neutron Research in support of this work.

Footnotes

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.1c03629.

Phonon mode calculations (ZIP)

Crystallographic data and Rietveld refinements, FTIR data, and information on the INS data and the phonon mode animations (PDF)

Complete contact information is available at: https://pubs.acs.org/10.1021/acs.jpcc.1c03629

The authors declare no competing financial interest.

CCDC 2079392–2079393 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing data_request@ccdc.cam.ac.uk, or by contacting Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Contributor Information

Jakob B. Grinderslev, Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, Aarhus University, Aarhus 8000, Denmark

Mikael S. Andersson, Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Göteborg SE-412 96, Sweden NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, United States.

Benjamin A. Trump, NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, United States

Wei Zhou, NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, United States.

Terrence J. Udovic, NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, United States Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742-2115, United States.

Maths Karlsson, Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Göteborg SE-412 96, Sweden.

Torben R. Jensen, Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, Aarhus University, Aarhus 8000, Denmark

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Phonon Animation Files
NIHMS1918225-supplement-Phonon_Animation_Files.zip (1.8MB, zip)
Supporting Info
NIHMS1918225-supplement-Supporting_Info.pdf (471KB, pdf)

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